The invention relates to a biogas plant fermenter comprising a vertically motor-adjustable, submersible motor stirrer.
In biogas plants, fermentation processes take place in which organic matter such as agricultural manure (liquid bovine manure, solid bovine manure, liquid porcine manure, solid porcine manure, liquid chicken manure, dry chicken manure) and/or agricultural residual matter (cut grass beet leaves, silages) and/or agro- or related industrial residual matter (brewer's grains, fruit residual matter, vegetable residual matter, rape meal, cereal residue, slops, molasses) is gasified as a biomass. The gases so generated collect in an upper fermenter tank region of a fermenter tank and can be immediately employed to generate energy, e.g. as a fuel gas for power generation in downstream internal combustion engines with electric generators. For the purpose of fermentation, liquids are added to the organic matter in the fermenter tank wherein the fermentation or gasification process takes place in aerobic or anaerobic conditions by means of microorganisms such as yeasts, bacteria, etc.
One problem arising in these generally known fermentation processes is that the biomass, in particular biomass solids, is not evenly distributed in the fermenter liquid as a rule since for example they rise, accumulating near the liquid surface area. On the other hand there may be the problem of biomass, in particular biomass solids having a higher specific weight than the fermenter liquids, settling down to the fermenter tank bottom where it inconveniently accumulates to form a grit layer. What is generally required for high efficiency of the fermentation process is a highly homogeneous biomass distribution in the fermenter liquid.
To achieve such a highly homogeneous biomass distribution in the fermenter liquid for enhancing efficiency, an agitator device in the relevant category has been known (DE 197 32 198 C1, FIG. 1). This agitator device comprises a submersible motor stirrer which by means of an electric motor-controlled rope winch is held to be height adjustable at an assembly holder arranged vertically in the tank. The submersible motor stirrer comprises a submersible motor having an approximately horizontal agitator shaft at which agitator blades are attached. It is further known from this document to pull out the submersible motor stirrer through a maintenance hole in the fermenter ceiling, up into a servicing well configured dome-shaped for maintenance and/or repairs by means of the rope winch. This is advantageously possible without having to lower the liquid level in the fermenter which would interfere with the fermentation process.
With the plant in operation the submersible motor stirrer is adjusted in height in relation to the stirring requirements by means of actuating the associated height adjustment motor, manually or else to a specified time schedule. The filling level of the fermenter may vary in relation to the course of the fermentation process in conjunction with a usually automated feeding of biomass. In this way, height adjustment of the submersible motor stirrer in the upper fermenter region may cause a state in which the agitator blades emerge at least in part from the substrate during stirring. This will cause the disadvantage of a reduced blending of the substrate and thus a diminished stirring effect, which will on the whole also reduce the biogas yield. Moreover, emerging agitator blades will spray the substrate beyond the substrate level which may soil for example inspection windows. With the agitator blades wholly immersed, the energy consumption of and mechanical load on the submersible motor stirrer are about consistent. In the emerging phase of an agitator blade or part of an agitator blade, however, the mechanical load on this agitator blade decreases and in re-immersing it increases again, such that the drive motor, the bearings and shafts will be subjected to periodically uneven loads. In particular in the case of a substrate having high solid biomass concentration this will increase wear on a submersible stirrer and may considerably reduce its service life.